1. Field of the Invention
This invention relates to a sensor for detecting moisture on a windshield, and in particular, to a sensor that automatically actuates a windshield wiper motor when water is detected on a windshield moisture sensor.
2A. Technical Considerations
The forward window of a vehicle, such as an automobile or aircraft, through which the operator views the outside world is generally referred to as a windshield. Vehicle windshields are generally of a laminated construction wherein multiple plies of glass and/or plastic are secured to one another by interlayer material. Because of the importance of maintaining a clear, undistorted viewing area through the windshield, wiper mechanisms are generally provided to clear selected portions of the windshield of water and/or dirt that may obscure the vehicle operator's vision.
Moisture sensors have been used to sense rain on windshields. The sensors may be connected to controls that automatically actuate the windshield wiper motors to remove the water and clear the vision area. The sensors generally include electrically conductive members with a protective coating on the exterior major surface of the windshield. The conduction members are arranged in a fixed relationship forming variable capacitors such that moisture on the protective coating alters the capacitance of the sensor's circuitry.
2B. Patents of Interest
U.S. Pat. No. 4,164,868 to Suntola teaches a capacitive humidity transducer wherein an electrically non-conductive base carries at least a pair of electrically conductive coatings which are spaced from each other along the major surface of the base. A dielectric film which is active with respect to water absorption is also carried by the base and covers at least a portion of the coatings. The dielectric film has a dielectric constant which varies as a function of the extent to which water has been absorbed by the film. An outer, electrically conductive, water-permeable layer is carried by the dielectric film. The dielectric film maintains the outer layer permanently out of contact with at least one of the coatings so that it is possible to measure a capacitance between these coatings, which will be indicative of atmospheric humidity.
U.S. Pat. No. 4,429,343 to Freud teaches a humidity-sensing element having two sets of interdigitated, thin film platinum fingers deposited on the surface of a glass substrate. The film is covered by a coating of water-absorbing material such as cellulose acetate butyrate or silicone rubber. The humidity sensitivity of the sensor arises from the humidity-related dielectric constant change which occurs in the coating over the fingers. As this dielectric constant changes, so does the capacitance between the interdigitated fingers.
U.S. Pat. No. 4,639,831 to Iyoda teaches a transparent sensor for detecting rain on window glass. The rain sensor is made from transparent material and is located within the wiping area on the exterior surface of the window glass. The sensor includes a pair of electrodes having interdigitated finger members that are insulated electrically from each other by a clearance. A transparent insulating protector film covers the electrodes and the clearance. The interdigitated members form capacitors having variable capacitance. When a drop of water is positioned on a portion of the protective coating defined between a pair of finger members, i.e., aligned with the clearance, the capacitance of the capacitor becomes greater than the normal capacitance because the dielectric constant of the drop of water is greater than the dielectric constant of air. Accordingly, when many drops of water are positioned on the protective coating, the total capacitance formed between the electrodes becomes larger.